I almost bought one of these today --
http://www.bikelightingsystem.com/vis360.html

Fairly light, recharges via USB (pretty convenient), both the taillight and headlight run off of one Li-ion battery that's contained in the taillight.

My only reservation, however, is about whether the rear light fits well enough on a helmet with a vertical back side like my Specialized. I'm guessing it'll be OK, but I'm also wondering if it will end up pointing at the ground or something like that, and I don't know how well the rear LEDs' light is dispersed.

Specialized's helmets:
http://www.specialized.com/us/en/bc/...jsp?spid=58627